1. Field of the Invention
The present invention relates to the selection of an operating mode of an integrated circuit from among several modes, in particular to definitively forbid or select a predetermined operating mode.
2. Discussion of the Related Art
Known integrated circuits include a device for selecting between a reserved mode providing access to all the circuit functionalities, and a user mode providing access to a limited number of the circuit functionalities. Such a device is used, for example, to initially start a circuit in a reserved mode, enabling testing of the circuit and storing of confidential information therein. The selection device can then be locked so that the circuit can only start in a user mode, whereby the stored confidential information cannot be read or modified.
FIG. 1 schematically shows an integrated circuit 2 (IC) intended for a secure application, for example supported by a smart card, and including a functional block 4 (FUNCT). Block 4 provides a control signal COM to an input terminal of a selection device 6 (SELECT) and receives a selection signal SEL from the output terminal of device 6. Device 6 includes a fuse 7 connected between a supply voltage Vdd and the output terminal of device 6, the melting of which is controlled by the reception of an active signal COM. The environment of circuit 2, connected to block 4 by an input/output access 8, is not shown.
Block 4 is configured to operate in the reserved mode if it receives an active signal SEL, that is, as long as fuse 7 has not melted, and in the user mode otherwise. An equivalent system can also use an anti-fuse which closes a circuit to make a function inactive. Signal COM is inactive as long as an operator does not control its activation. Initially, that is, after manufacturing of the circuit, fuse 7 has not melted and block 4 operates in the reserved mode. An operator can test and program block 4 via access 8. Once the testing and the programming of block 4 are complete, the operator controls block 4 to provide an active signal COM to the input terminal of device 6 to melt fuse 7 and cause a definitive selection of the user mode.
Fuse 7 is configured in the integrated circuit so as to forbid a user from reactivating the reserved mode by a keying of signal SEL. Unfortunately, such a fuse may under certain conditions form again after having melted. The non-volatile character of the programming of the state of signal SEL performed by the fuse is then lost.
Further, controlling the melting of a fuse requires a significant power, which requires block 4 or device 6 to include power elements. Such elements increase the circuit size and power consumption.
Moreover, the forming of fuses in an integrated circuit requires certain manufacturing steps, for example, steps of deposition of a tunnel oxide, which are not available in all integrated circuit manufacturing processes, for example, those of integrated circuits with a ferromagnetic memory. It is thus not currently possible to form a circuit with a ferromagnetic memory provided with an operating mode selection device such as in FIG. 1.